On-site disposal of waste water (sewage) from single family residences in areas with no conventional sewer system has conventionally been accomplished by a septic tank system. The anaerobic effluent discharged from the septic tank, after settling of the solids portion of the incoming waste water, is passed into a subsurface drain field for percolation into the surrounding soil. Such a system works satisfactorily if properly installed and if proper soil conditions for disposal of the effluent by the drain field exist. While most residential septic systems work with harmony between the septic tank and drain field, there are increasing numbers of drain field failures.
In a septic system waste material is treated by an anaerobic bacterial process which slowly decomposes the sewage into hydrogen sulfide gas, methane and water. Aside from the fact that such systems are relatively slow working, the gases resulting from the decomposition process have noxious odors and the methane gas is highly inflammable. Additionally, the slow decomposition process associated with septic systems will generally cause solids to accumulate in the system at a fairly high rate, since they are decomposed at a slower rate than they are deposited in the system. This results in the necessity of relatively frequent cleaning of the system. It will also be found that in some instances soil characteristics are such that the usual septic system cannot be used satisfactorily.
In such systems, heretofore, small treatment plants which make use of chemical and/or biological treatment schemes to render the effluent suitable for disposal have been suggested. These treatment plants usually are designed to replace the conventional septic system. Such treatment plants are generally too prohibitively expensive to be not economically feasible for treatment of residential sewage. Of these less common types of residential sewage treatment system for single dwelling units are those that operate on an aerobic bacterial process. In this type of system waste materials are broken down into carbon dioxide and water. In addition to the fact that the aerobic process is much more rapid than the anaerobic process, the products or the process are not objectionable with regard to odor or flammability.
Despite the advantages of aerobic sewage treatment systems, the majority of residences requiring their own sewage treatment still rely upon septic systems. In addition to the fact that a septic system may be less expensive initially to install, the apparent preference for septic systems may be based to some extent upon the necessity or providing proper operating conditions to support the bacteria growth necessary to provide an aerobic decomposition of waste materials. Although the maintenance of the necessary conditions to support aerobic bacterial activity may present no more than routine problems for a commercial or municipal sewage treatment plant, such systems must necessarily be relatively maintenance free and simple to operate in order to obtain acceptance on a wide scale basis for domestic, as opposed to commercial or municipal, use. Additionally, previous systems of this type have incorporated individual filter systems or small area filter systems which do not provide for adequate length of time between cleaning or take into consideration surge conditions that occur during normal operating conditions.